Battery cell monitoring and balancing circuit

ABSTRACT

A monitoring circuit for accurately monitoring a voltage level from each of a plurality of battery cells of a battery pack includes an analog to digital converter (ADC) and a processor. The ADC is configured to accept an analog voltage signal from each of the plurality of battery cells and convert each analog voltage signal to a digital signal representative of an accurate voltage level of each battery cell. The processor receives such signals and provides a safety alert signal based on at least one of the signals. The ADC resolution may be adjustable. A balancing circuit provides a balancing signal if at least two of the digital signals indicate a voltage difference between two cells is greater than a battery cell balance threshold. An electronic device including such monitoring and balancing circuits is also provided. Various methods are also provided.

RELATED APPLICATION

This application is a continuation of U.S. patent application Ser. No.11/459,473, filed Jul. 24, 2006, now U.S. Pat. No. 7,696,725, which is adivisional application of U.S. patent application Ser. No. 10/464,973,filed Jun. 19, 2003, now U.S. Pat. No. 7,081,737.

FIELD OF THE INVENTION

The present invention relates to a battery cell monitoring and balancingcircuit, and in particular to such a circuit that directly digitizesanalog battery cell voltage levels into associated digital signals.

BACKGROUND OF THE INVENTION

Multi-cell rechargeable batteries are utilized in a variety ofapplications given their higher voltage delivery and greater capacity.Such applications include, but are not limited to, electronic devicessuch as laptops, cellular phones, personal digital assistants, and thelike. Certain types of battery cells, e.g., lithium ion cells, can behazardous if charged significantly above its normal charge range ordischarged below its normal charge range. As such, a typical monitoringand protection circuit may utilize a switch network to transfer voltagecharges to a capacitor. Voltage on the capacitor then represents thebattery cell voltage and may then be provided to a plurality ofcomparators for comparing the voltage to various threshold levels suchas over voltage and under voltage levels.

There are several drawbacks to such an arrangement. First, such anarrangement may provide for unreliable voltage measurements. Forinstance, if the current through a particular cell is not constant orthe cell voltage fluctuates because of the internal resistance of thecell or some other factors, the sampled voltage may not be trulyindicative of the voltage on the cell. As such, corrective measures maybe incorrectly taken based on such erroneous measurements.

Second, the threshold levels such as over voltage and under voltage arenot easily adjustable. This is an issue since different battery packtypes and different manufacturers for the same battery pack type mayrequire different over and under voltage thresholds. For example, onebattery pack manufacture may require a 3.0 volt under voltage thresholdwhile another may require 2.5 volts for the same battery pack type.Third, when the cells in one battery pack are unbalanced (e.g., aftermany charging and discharging cycles) a traditional bleeding method canbe undertaken to balance the cell. However, a bleeding decision istypically made only when the battery is near fully charged at the timeof charging. Since bleeding current is typically limited in order toavoid excessive heat generation, bleeding takes a certain time interval.If more than one cell needs to be bled, there is simply not enough timein one charge cycle to accomplish this task.

Accordingly, there is a need in the art for a cell monitoring andbalancing circuit that overcomes these and other deficiencies in theprior art.

BRIEF SUMMARY OF THE INVENTION

A monitoring circuit for accurately monitoring a voltage level from eachof a plurality of battery cells of a battery pack consistent with theinvention includes an analog to digital converter (ADC) and a processor.The ADC is configured to accept an analog voltage signal from each ofthe plurality of battery cells and convert each analog voltage signal toa digital signal representative of a voltage level of each battery cell.The processor is configured to receive each digital signal and toprovide a safety alert signal based on at least one of the digitalsignals.

According to another embodiment of the invention, a balancing circuit isprovided. The balancing circuit includes an ADC configured to accept ananalog voltage signal from each of a plurality of battery cells of abattery pack and to convert each analog voltage signal to a digitalsignal representative of a voltage level of each battery cell. Thebalancing circuit also includes a processor configured to receive eachdigital signal and to provide a balance signal if at least two of thedigital signals are representative of a voltage difference between afirst voltage level of a first battery cell and a second voltage levelof a second battery cell greater than a battery cell balance threshold.

In yet a further embodiment of the invention, an electronic devicecapable of being powered by a battery pack having a plurality of batterycells is provided. The electronic device includes a monitoring circuitfor accurately monitoring a voltage level from each of the plurality ofbattery cells. The monitoring circuit includes an ADC configured toaccept an analog voltage signal from each of the plurality of batterycells and convert each analog voltage signal to a digital signalrepresentative of a voltage level of each battery cell. The monitoringcircuit also includes a processor configured to receive each digitalsignal and to provide a safety alert signal based on at least one of thedigital signals.

In still a further embodiment of the invention, an electronic devicecapable of being powered by a battery pack having a plurality of batterycells is provided. The electronic device includes a balancing circuit,where the balancing circuit includes and ADC configured to accept ananalog voltage signal from each of the plurality of battery cells andconvert each analog voltage signal to a digital signal representative ofa voltage level of each battery cell. The balancing circuit alsoincludes a processor configured to receive each digital signal and toprovide a balance signal if at least two of the digital signals arerepresentative of a voltage difference between a first voltage level ofa first battery cell and a second voltage level of a second battery cellgreater than a battery cell balance threshold.

In still another embodiment of the invention, there is providedbalancing circuit including an ADC configured to accept an analogvoltage signal from each of a plurality of battery cells of a batterypack and convert each analog voltage signal to a digital signalrepresentative of a voltage level of each battery cell. The balancingcircuit further includes a processor configured to receive each digitalsignal and to provide a pre-balance signal during a second mode before avoltage difference between a first voltage level of a first battery celland a second voltage level of a second battery cell exceeds a batterycell balance threshold if the voltage difference existed during a firstmode.

In another embodiment, a method of balancing voltage levels for aplurality of battery cells of a battery pack is provided. The methodincludes the steps of: converting a first analog voltage signal from afirst cell to a first digital signal representative of the first analogvoltage signal; converting a second analog voltage signal from a secondcell to a second digital signal representative of the second analogvoltage signal; determining a difference between the first voltage leveland the second voltage level; comparing the difference to a battery cellbalance threshold; and providing a balance signal if the difference isgreater than the battery cell balance threshold.

BRIEF DESCRIPTION OF THE DRAWINGS

Advantages of the present invention will be apparent from the followingdetailed description of exemplary embodiments thereof, which descriptionshould be considered in conjunction with the accompanying drawings, inwhich:

FIG. 1 is a simplified high level block diagram of an electronic devicethat may receive power from a battery pack having a plurality of batterycells, where the electronic device has a cell monitoring and balancingcircuit consistent with the present invention;

FIG. 2 is a block diagram of a monitoring circuit that may be utilizedin the electronic device of FIG. 1;

FIG. 3A is a block diagram of a balancing circuit that may be utilizedin the electronic device of FIG. 1;

FIG. 3B is an exemplary plot of the discharge characteristics of twobattery cells that may receive a pre-balancing signal from the balancingcircuit of FIG. 3A;

FIG. 4 is a more detailed block diagram of an exemplary cell monitoringand balancing circuit consistent with the invention; and

FIG. 5 is an exemplary circuit diagram of one bleeding circuit that maybe utilized in the bleeding network circuit of FIG. 4.

DETAILED DESCRIPTION

Turning to FIG. 1, a simplified block diagram of an electronic device100 capable of being powered from a battery pack 102 or a DC powersource 104 is illustrated. The battery pack 102 may containing aplurality of battery cells 102-1, 102-2, 102-n. The cell types may be ofvarious rechargeable types known in the art such as lithium-ion,nickel-cadmium, nickel-metal hydride batteries, or the like.

If the electronic device 100 is a laptop computer it would include avariety of components known to those skilled in the art which are notillustrated in FIG. 1. For example, the laptop may include an inputdevice for inputting data to the laptop, a central processing unit (CPU)or processor, for example a Pentium processor available from IntelCorporation, for executing instructions and controlling operation of thelaptop, and an output device, e.g., a LCD or speakers, for outputtingdata from the laptop.

To recharge the battery pack 102 and/or supply power to the system 112,a DC power source 104 may be coupled to the device 100. The DC powersource 104 may be an AC/DC adapter which is configured to receiveconventional 120 volts AC from a wall outlet and convert it to a DCoutput voltage. The DC power source 104 may also be a DC/DC adapter suchas a “cigarette lighter” type adapter configured to plug into that typeof socket. Such a DC power source 104 is illustrated in FIG. 1 asseparate from the device 100, but it may be built into some devices. Theelectronic device 100 may also have a power supply block 110. Ingeneral, the power supply block 110 includes various components tomonitor, control, and direct power from each power source 102, 104 toeach other and to the system 112 of the electronic device 100 undervarious conditions.

Advantageously, the electronic device 100 includes a cell monitoring andbalancing circuit 108 as further detailed herein. The cell monitoringand balancing circuit 108 is shown separate from the power supply block110 for clarity, but it may be included as part of the power supplyblock 110. The cell monitoring and balancing circuit 108 may function asa monitoring circuit, a balancing circuit, or both as further detailedherein. The cell monitoring and balancing circuit 108 provides digitalsignals representative of the voltage level of each cell 102-1, 102-2,102-n to various components of the device such as a battery gas gauge118. The battery gas gauge may utilize such signals to provide an outputsignal representative of the remaining useful life of the battery pack102.

Turning to FIG. 2, a block diagram of an exemplary monitoring circuit208 that may be utilized as the monitoring portion of the cellmonitoring and balancing circuit 108 of FIG. 1 is illustrated. Themonitoring circuit 208 generally includes an analog to digital converter(ADC) 220 and a processor 222. The ADC 220 is configured to accept ananalog voltage signal from each of the battery cells 102-1, 102-2, 102-nand convert each to a digital signal. The processor 220 is configured toreceive each digital signal and provide a safety alert signal based onat least one of the digital signals.

Advantageously, the ADC 220 may function as an “averaging” type ADC totake an “average” reading of the voltage of each battery cell 102-1,1-2-2, 102-n such that transient deviations from normal readings willnot adversely impact the quality of the digital signal representation ofthe analog signal. For example, such transient deviations could includevoltage spikes or other rapid voltage fluctuations that can occur due toa number of factors including varying charging currents or load currentspassing through the internal resistance of the cells.

The ADC 220 may include one or more various types of ADCs to function asan averaging type ADC. For instance, the ADC 212 may include asingle-slope integrating ADC, a dual-slope integrating type ADC, or asigma-delta type ADC to name a few. Such a sigma-delta type ADCgenerally includes an analog modulator portion that essentiallydigitizes an input analog signal at very high sampling frequency equalto Fs×OSR, where Fs is the Nyquist Frequency and OSR is the oversampling ratio to the Nyquist Frequency. The outputs from this oversampling may be combined in groups and the groups may be averaged. Ananalog signal representing the voltage of any particular battery cell102-1, 102-2, 102-n may thus be sampled many times, e.g., in thethousands of times in some instances. As such, a few incorrect samplingsof transient deviations will have little effect on the average signalconverted by the averaging type ADC 220 to an associated digital signal.

In addition, the ADC 220 may also have an adjustable resolutiondepending on a particular need. For instance, the processor 222 mayinstruct the ADC 220 via data path 217 to utilize a desired resolutionlevel for a particular conversion of an analog signal from a particularbattery cell 102-1, 102-2, 102-n to an associated digital signal. Theresolution may be adjusted to a relatively higher resolution insituations where there is more sensitivity to the analog voltagemeasurement. For example, such a situation requiring higher resolutionmay be for open circuit voltage detection.

In contrast, resolution may be adjusted to a relatively lower resolutionin situations where there is less sensitivity to the analog voltagemeasurement. For example, such a situation requiring lower resolutionmay be for under-voltage detection. The lower the resolution, the lesstime is needed to complete one valid analog to digital conversion. Inone of many examples, a relatively higher resolution may require 15 bitsof data while the relatively lower resolution may only require 10 bitsof data. Those skilled in the art will recognize the actual number ofbits may vary depending on the particular requirements for the digitaldata and the resolution capabilities of the ADC 220.

To achieve a desired resolution value, the ADC 220 may be of any type ofADC that can adjust resolution upon direction from the ADC controlsignal from the processor 222. For instance, the ADC 220 may be asigma-delta type as earlier detailed. Such a sigma-delta modulator typecan adjust resolution based, in part, on the OSR. In general, higherresolution can be obtained with a higher OSR.

The ADC 220 may also be a successive approximation type ADC. In general,a successive approximation type ADC conceptually uses a singlecomparator to make its conversion. If N bits of resolution are desiredas indicated by the control signal from the processor 220, a successiveapproximation type ADC would make N comparator operations to achieve Nbits of resolution. Other types of ADCs such as a single-slope ordual-slope integrating type ADC, and or combinations of various types ofADCs may be used in an ADC 220 consistent with the invention to achievean adjustable resolution.

The processor 222 is configured to receive digital signals from the ADC220 representative of the voltage level of each battery cell 102-1,102-2, 102-n and to provide a safety alert signal based on at least oneof the digital signals. The safety alert signal may be a charge alertsignal or a discharge alert signal.

During charging of the battery pack 102 by the DC power source 104, itis important to monitor the voltage level of each cell in order toprotect against over voltage conditions. This is because certain typesof electrolyte battery cells, such as lithium ion cells, are susceptibleto damage if charged above their normal threshold level. As such, theprocessor 222 sends a charge alert signal if the digital signals fromthe ADC 220 indicate that at least one of the cells 102-1, 102-2, 102-nhas a voltage level greater than an over voltage threshold value over apredetermined time interval. As such, some preventative action may thenbe taken such as to stop charging. Alternatively, the processor 222 mayalso provide the charge alert signal if one of the digital signals fromthe ADC 220 indicates that one cell has a voltage level greater than theover voltage threshold level while the battery pack 102 is beingcharged.

During discharging of the battery pack 102, the monitoring circuit 208may utilize a plurality of under voltage threshold levels to preventdamage to the cells and to provide adequate warnings to a user of anassociated electronic device 100. For instance, the processor 222 maysend a discharge alert signal if the digital signals from the ADC 220indicate that at least one of the cells 102-1, 102-2, 102-n has avoltage level lower than an under voltage threshold value over apredetermined time interval while the battery pack 102 is beingdischarged. As such, some preventative action may then be taken such asto stop power supply from the battery pack. Alternatively, the processor222 may also provide the discharge alert signal if one of the digitalsignals from the ADC 220 indicates that one cell has a voltage levelless than the under voltage threshold level while the battery pack 102is being discharged.

In addition to the under voltage threshold level, other under voltagethreshold voltage levels greater than the under voltage threshold levelmay also be utilized by the processor 222 to provide advanced notice ofa potentially impending low voltage condition. For instance, if theelectronic device 100 is a laptop computer and the under voltagethreshold level is reached without any notice to the user, a user maylose a significant amount of important unsaved data.

Therefore, a first under voltage threshold level may be programmed intoand stored in any applicable memory device of the system 100. As theprocessor 222 receives digital signals from the ADC 220 representativeof the voltage levels of each cell, the processor 222 can compare suchvoltage levels to the first under voltage threshold level. If thevoltage level on one of the cells drops below such level, the processor222 can provide an appropriate signal via path 290 to other componentsof the system 100. As such, an alert message may then be provided to auser of the electronic device 100. The first under voltage thresholdlevel may be chosen based on the system 100 particulars including thetime required to perform typical tasks and the power required for suchtasks.

For instance, if the electronic device 100 is a laptop computer thefirst threshold level may be chosen high enough that upon notice of alow power condition (e.g., one cell of the cells 102-1, 102-2, 102-n hasa voltage level less than the first threshold level) a user still hasenough power and time to operate the laptop for an additional timeperiod. Another second threshold level, which is less than the firstthreshold level, may also be utilized to indicate that a shorter timeperiod is available to the user for proper operation. For instance, ifthe any cell voltage is less than the second threshold level, anotheralert signal may indicate to the user that there is likely only enoughtime for saving and shutting down of the laptop computer before theunder voltage threshold level is reached and battery power to the systemis halted.

Advantageously, all threshold levels are adjustable by the processor222. For instance, the over and under voltage threshold level may beadjusted based on the particular type of cell utilized. The over andunder voltage threshold values may be stored in any variety ofelectronic storage media in the device. For instance, the processor 222may have internal registers 230 that could store such threshold levels.

In addition, the over and under voltage threshold level can be adjustedbased on other parameters that affect the charging and dischargingperformance of the cells such as ambient temperature and age of thecells. Ambient temperature information may be provided to the processor222 by a temperature sensor 292.

In addition, a sampling time interval is also adjustable by theprocessor 222. A sampling time interval includes that time period inwhich all cells are sampled once and a valid digital conversion is madeby the ADC 220 for each cell. This enables the processor to sample thecells more frequently during certain conditions, e.g., during charging,when more frequent sampling is advantageous. This also enables theprocessor to sample the cells less frequently during other conditions,e.g., the battery pack 102 is in sleep or idle mode, when less frequentsampling is adequate. For example, such a sampling time interval may beonce per minute when the battery pack is in sleep or idle mode. As such,the ADC 220 may also be placed in a sleep mode by the processor 222 inorder to conserve energy when it is not needed to make a conversion.

Turning to FIG. 3A, a block diagram of an exemplary balancing circuit308 is illustrated that may be utilized in the electronic device ofFIG. 1. The balancing circuit 308 generally includes an ADC 320 andprocessor 322 as previously detailed with respect to FIG. 2. Inaddition, the processor 322 controls either the bleeding network circuit340, the charge shuttling circuit 342, or both to balance voltage levelsof the cells 102-1, 102-2, 102-n as further detailed herein.

The processor 322 receives associated digital signals from the ADC 320as previously detailed that are representative of an accurate voltagereading for each cell 102-1, 102-2, 102-n. As such, the processor 322not only knows which cell has the highest voltage level and the lowestvoltage level, it also knows the magnitude of the voltage differencebetween such cells and the voltage level of each cell. The processor 322utilizes this information from the ADC 320 to make intelligent decisionsregarding cell balancing as detailed herein that result in accurate andfast cell balancing.

First, cell balancing decisions may be made at any time (during chargingmode, discharging mode, or even during idle mode) whenever there is avoltage difference between the highest voltage cell and the lowestvoltage cell that is greater than some battery cell balance threshold.During charging, cell balancing is useful to control the cells with thehigher voltage levels to enable the lower voltage level cells time tocatch up. Since battery charging is typically limited by any one cellreaching its end of charge voltage level, this enables each cell toapproach its end of cell voltage level. Otherwise, a cell that reachedits end of voltage level charge quickly would prevent other cells frombeing more fully charged.

Since the processor 322 is repeatedly receiving updated accurate voltageinformation of each cell 102-1, 10202, 102-n from the ADC 320, it caninstruct the bleeding network circuit 340 to conduct appropriate cellbleeding or it can instruct the charge shuttling circuit 342 to conductappropriate charge transfers as soon as it detects a voltage differencebetween a higher voltage cell and a lower voltage cell is greater than apredetermined battery cell balance threshold.

Balancing can occur by providing a bleeding current to one or more ofthe cells having higher voltage levels during certain time intervals.Such time intervals may overlap such that bleeding for more than onecell may occur at similar times. In addition, the starting times for thebleeding of two or more cells can start at substantially the same timein order to speed up the bleeding process. Furthermore, cell bleedingcan be adjusted based on the difference in voltage between the higherand lower voltage cells. As such, a first cell with a relatively highervoltage level may be bled by a higher bleeding current than a secondcell with a voltage level slightly less than the first cell.

In general, the greater the difference between the higher voltage celland lower voltage cell, the greater the bleeding current provided by thebleeding network circuit 340. The upper limit of such bleeding currentis typically limited by heat dissipation concerns. Also, the less thedifference between the higher voltage cell and the lower voltage cell,the less bleeding current is provided by the bleeding network circuit340.

Balancing cell voltage levels can occur not only by bleeding the cellswith the higher voltage levels, but also by shuttling charges fromhigher voltage cells to lower voltage cells. Such charge shuttling iscontrolled by the processor 322 which provides appropriate controlsignals to the switch network 350 to control the shuttling of chargefrom cells with higher voltage levels to cells with lower voltage levelsby utilizing the charge shuttling circuit 342.

The processor 322 knows which one or more cells 102-1, 102-2, 102-n havehigher voltage readings compared to the others due to the digitalsignals received from the ADC 320. As such, the processor 322 may directthe appropriate switches in the switch network 350 to close such thatone or more cells with a higher voltage level transfers some chargelevel to the charge shuttling circuit 342. The processor 322 may thenfurther direct appropriate switches in the switch network 350 to closeto transfer charges from the charge shuttling circuit 342 to the cellwith a lower voltage level. Such a charge transfer process stops whenthe processor 322 instructs such process to stop, e.g., when theprocessor knows that an appropriate balanced voltage level is obtainedbetween appropriate cells.

The processor 322 may also provide a pre-balance signal to the bleedingnetwork circuit 340 or the charge shuttling circuit 342 to start abalancing process before an imbalance exists. The pre-balance signal maybe provided in a second mode if a cell imbalance occurred in a firstmode occurring before the second mode. The first mode may be a firstcharging mode or a first discharging mode. The second mode may be asecond charging mode or a second discharging mode. For instance, theprocessor 322 has each cell's voltage information in digital form andknows the cells voltage history. After one or more modes, e.g., chargingand discharging cycles, if the processor 322 notices that an imbalancealways happens to one specific cell, e.g., if one cell has a lowervoltage level towards the end of discharge, then the processor can starta balancing process during a later mode before a noticeable voltagedifference is even detectable.

For instance, turning to FIG. 3B, an exemplary plot of capacity (Ah) ofa lithium ion cell at 21 degree Celsius versus cell voltage isillustrated for an exemplary Cell A 303 and Cell B 305. As illustrated,it is not until near the end of discharge near the voltage drop knee 307that the voltage difference between Cell A and Cell B becomesappreciable. If balancing waited until such an appreciable voltagedifference, then there is little time to perform balancing. If however,the processor 322 knows that cell A typically has a lower voltage thancell B towards the end of discharge from at least one prior dischargecycle or mode, the processor 322 can start balancing for cell A early ina later discharge cycle or mode without waiting for the appreciablevoltage difference to occur. For instance, the processor may instructcharges from Cell B and/or other cells to be transferred to the chargeshuttling circuit 342 and then to Cell A early in the later dischargemode. In this way, Cell A may be balanced with Cell B so that noappreciable voltage difference exists between the cells near their endof discharge cycle.

Turning to FIG. 4, an exemplary cell monitoring and balancing circuit408 is illustrated. The cell monitoring and balancing circuit 408includes the functionality of the monitoring circuit 208 of FIG. 2 andthe balancing circuit 308 of FIG. 3. In general, the circuit 408 mayinclude an ADC 420, a processor 422, a switch network control circuit451, a switch network 450, a bleeding network circuit 440, a drivercircuit 427, and a protection circuit 429.

Individual analog cell voltage levels for each cell of the battery pack402 may be sampled directly through the switch network 450. The sampledanalog signals may then be converted into associated digital signals bythe ADC 420 as previously detailed. For instance, when the first cell402-1 is to be sampled, the switches 450 a and 450 c of the switchnetwork 450 close, while all other switches of the switch network 450remain open. As such, the positive terminal of the first cell 402-1 iscoupled through switch 450 a to the positive input terminal of the ADC420. Also, the negative terminal of the first cell 402-1 is coupledthrough switch 450 c to the negative input terminal of the ADC 420. Allswitches of the switch network 450 will remain in these positions untila valid analog to digital conversion is completed by the ADC 420 over agiven conversion time period for the first cell 402-1.

Similarly the second cell 402-2 (through closed switches 450 b and 450 ewith other switches of the switch network 450 open), the third cell402-3 (through closed switches 450 d and 450 g with other switches ofthe switch network 450 open), and the fourth cell 402-4 (through closedswitches 450 f and 450 i with other switches of the switch network 450open) may also be directly coupled to the ADC 420 in a like manner fordirect sampling of each cell 402-2, 402-3, and 402-4.

The charge shuttling circuit 442 may include an energy storage elementsuch as a transformer, inductor, or capacitor. In the illustratedembodiment, a capacitor 443 is utilized as the energy storage element.If charge shuttling among cells is directed by the processor 422, theappropriate switches 450 a to 450 g of the switch network 450 directcharges from one or more of the cells with the higher voltage to betemporarily stored in the capacitor 443. Such charges are then shuttledto a lower voltage cell by the appropriate switches of the switchnetwork 450. The processor 422 controls the switch network 450 via theswitch network control circuit 451.

An ADC 420 consistent with the invention may also be calibrated for eachindividual cell 402-1, 402-2, 402-3, and 402-4 in order to compensatefor any offset. Such offset can be due to any number of factors such asdifferent voltage gradients, and switching charge injection fordifferent ADC cell channels. For instance, to calibrate the first cell402-1, switches 450 b and 450 c would be closed with all other switchesof the switch network 450 open. As such, the input terminals of the ADC420 are connected to the virtual ground of the first cell 402-1. The ADC420 would then convert such analog signal into an associated firstoffset digital signal. The first offset digital signal may then bestored in any available memory device.

Similarly, to calibrate the second cell 402-2, switches 450 d and 450 ewould be closed. To calibrate the third cell 402-3, switches 450 f and450 g would be closed. Finally, to calibrate the fourth cell 402-4,switches 450 h and 450 i would be closed. As such, four offset valuesfor each associated cell 402-1, 402-2, 402-3, and 402-4 can be obtainedand stored. When the ADC 420 is later converting an analog measurementfor an associated cell, the processor 422 can instruct acquisition ofoffset-free data by subtracting the stored associated offset value forthe associated cell. As such, accurate measurement of the analog signalfrom each cell is further promoted.

A protection circuit 429 may also be incorporated into the cell balanceand monitoring circuit 408 in order to monitor the current flowing into(charging mode) or out of (discharging mode) the battery pack 402 forvarious power crisis conditions, e.g., over current or short circuitconditions, and alert the processor 422 of such conditions so thatpreventative action can be taken. For instance, a current sensingelement such as sense resistor 491 may be coupled to the battery pack402 to provide the protection circuit 429 with a signal representativeof the current level to or from the battery pack as that current levelvaries. If the current level is greater than a first current thresholdlevel, (e.g., the voltage drop across the sense resistor 491 is greaterthan the first current threshold level times the value of the senseresistor), then the protection circuit 429 provides an over currentalert signal to the processor 422 via data path 437.

The current level may be greater than a second current threshold level,where the second current threshold level is greater than the firstcurrent threshold level. In this instance (e.g., the voltage drop acrossthe sense resistor is greater than the second current threshold leveltimes the value of the sense resistor), the protection circuit 429provides a short circuit alert signal to the processor via data path439.

In response to such control signals from the protection circuit 429, theprocessor 422 makes decisions and sends appropriate control signals suchthat appropriate power safety measures are taken. The processor 422 mayprovide an appropriate control signal to the switch driver 427 in orderto drive the discharge switch Q1 open. Alternatively, the processor 422may also provide a message to a host component via data path 490 suchthat some alternative component, e.g., a power management unit, maymanage any necessary corrective action to ensure power supply safety.

The bleeding network circuit 440 may include a plurality of bleedingcircuits 440-1, 440-2, 440-3, and 440-4 to provide an adjustablebleeding current to each associated cell 402-1, 402-2, 402-3, and 404-4as previously detailed.

Turning to FIG. 5, one exemplary bleeding circuit 500 is illustrated.The bleeding circuit is responsive to a digital control signal from theprocessor 422 to control the bleeding current from the associated cell402-1, 402-2, 402-3, and 404-4 as previously detailed. The bleedingcircuit 500 may include a plurality of switches S0, S1, S(N−1) and anassociated plurality of resistive elements, e.g., resistors, havingvarying resistive values R, R/2, and R/(N−1). In one embodiment, theswitches may be MOSFET type transistors having their control or gateterminal configured to receive the digital control signal from theprocessor 422. An N-bit digital control signal from the processor 422may then dictate which switch S0, S1, S(N−1) turns ON and hence whichresistive value R, R/2, or R/(N−1) is coupled in parallel with theassociated battery cell 402-1, 402-2, 402-3, and 404-4, where theR/(N−1) resistive value is less than the R/2 resistive value, and theR/2 resistive value is less than the R resistive value.

If a large bleeding current is desired, the N-bit digital control signalfrom the processor 422 may instruct a second switch, e.g., switchS(N−1), to close such that a lower resistive value, e.g., R/(N−1), iscoupled in parallel with the associated cell. As such, a larger bleedingcurrent more quickly lowers the voltage level of the higher voltagecell. If a relatively smaller bleeding current is desired, the N-bitcontrol signal from the processor 422 may instruct a first switch, e.g.,switch S1, to close such that a relatively higher resistive value, e.g.,R, is coupled in parallel with the associated cell. As such, arelatively smaller bleeding current is provided to more slowly lower thevoltage level of a higher voltage cell.

The cell bleeding control signals from the processor 422 may also beprovided to a battery gas gauge in order to provide such gauge withaccurate cell bleeding information. For instance, a N-bit controlsignals provided to the cell bleeding circuits 440-1, 440-2, 440-3,440-4 may also be provided to associated battery gas gauge. As such, thegauge also knows which cell 402-1, 402-2, 402-3, 402-4 is being bled andthe associated bleeding current level for each cell. Therefore, thegauge can make more reliable calculations to determine the remainingamount of battery life taking into account bleeding current levels forcharging capacity calculations.

The embodiments that have been described herein, however, are but someof the several which utilize this invention and are set forth here byway of illustration but not of limitation. It is obvious that many otherembodiments, which will be readily apparent to those skilled in the art,may be made without departing materially from the spirit and scope ofthe invention as defined in the appended claims.

1. An electronic device capable of being powered by a battery packhaving a plurality of battery cells, said electronic device comprising:a monitoring circuit for accurately monitoring a voltage level from eachof said plurality of battery cells, said monitoring circuit comprising:an analog to digital converter (ADC) configured to accept an analogvoltage signal from each of said plurality of battery cells and converteach said analog voltage signal to a digital signal representative of avoltage level of each said battery cell; and a processor configured toreceive each said digital signal and to provide a safety alert signalbased on at least one of said digital signals.
 2. The electronic deviceof claim 1, wherein said safety alert signal is a charging alert signalif at least one of said digital signals is representative of a voltagelevel of one of said battery cells greater than an over voltagethreshold value during a predetermined monitoring time interval whilesaid plurality of battery cells are being charged.
 3. The electronicdevice of claim 2, wherein said charging alert signal is provided if oneof said digital signals is representative of a voltage level of one ofsaid battery cells greater than said over voltage threshold value whilesaid plurality of battery cells are being charged.
 4. The electronicdevice of claim 2, wherein said over voltage threshold level isadjustable by said processor.
 5. The electronic device of claim 4,wherein said over voltage threshold level is adjustable by saidprocessor based on ambient temperature.
 6. The electronic device ofclaim 1, wherein said safety alert signal is a discharge alert signal ifat least one of said digital signals is representative of a voltagelevel of one of said battery cells less than an under voltage thresholdvalue during a predetermined monitoring time interval while saidplurality of battery cells are being discharged.
 7. The electronicdevice of claim 6, wherein said discharge alert control signal isprovided if one of said digital signals is representative of a voltagelevel of one of said battery cells less than said under voltagethreshold value while said plurality of battery cells are beingdischarged.
 8. The electronic device of claim 6, wherein said undervoltage threshold level is adjustable by said processor.
 9. Theelectronic device of claim 8, wherein said over under voltage thresholdlevel is adjustable by said processor based on ambient temperature. 10.The electronic device of claim 1, wherein said ADC is an averaging typeADC.
 11. The electronic device of claim 10, wherein said averaging typeADC samples a voltage level for each said battery cell a plurality oftimes over a sampling period before converting said analog voltagesignal to an associated digital signal.
 12. The electronic device ofclaim 1, wherein said ADC has a resolution, and said resolution isadjustable by said processor.
 13. The electronic device of claim 1,wherein said monitoring circuit further comprises a switch networkcoupled between said plurality of battery cells and said ADC, saidswitch network having a plurality of switches controlled by saidprocessor, said switch network enabling each of said plurality ofbattery cells to be directly coupled to said ADC through said switchnetwork.
 14. The electronic device of claim 1, wherein said ADC has apositive input terminal and a negative input terminal, wherein a firstoffset value for a first battery cell being is determined by couplingone terminal of said first battery cell to both said positive and saidnegative input terminal of said ADC, wherein said processor is furtherconfigured to apply said first offset value to a first digital signalrepresentative of a voltage level of said first battery cell therebyproviding a first calibrated digital signal.
 15. The electronic deviceof claim 14, wherein a second offset value for a second battery cell isdetermined by coupling one terminal of said second battery cell to bothsaid positive and said negative input terminal of said ADC, wherein saidprocessor is further configured to apply said second offset value to asecond digital signal representative of a voltage level of said secondbattery cell thereby providing a second calibrated digital signal. 16.The electronic device of claim 1, wherein said ADC samples a voltagelevel of each said battery cell once every sampling time period, whereinsaid sampling time period is adjustable by said processor.
 17. Theelectronic device of claim 16, wherein said sampling time period has afirst time interval during a first condition, and said sampling timeperiod has a second time interval during a second condition, said secondtime interval being longer than said first time interval and said secondcondition being a sleep mode of said plurality of battery cells.
 18. Theelectronic device of claim 6, wherein said safety alert signal is afirst discharge alert signal if at least one of said digital signals isrepresentative of a voltage level of one of said battery cells less thana first under voltage threshold level during said predeterminedmonitoring time interval while said plurality of battery cells are beingdischarged, wherein said first under voltage threshold level is greaterthan said under voltage threshold level.
 19. The electronic device ofclaim 18, wherein said safety alert signal is a second discharge alertsignal if at least one of said digital signals is representative of avoltage level of one of said power sources less than a second undervoltage level during said predetermined monitoring time interval whilesaid plurality of power sources are being discharged, wherein saidsecond under voltage threshold level is less than first under voltagethreshold level and greater than said under voltage threshold level. 20.The electronic device of claim 1, wherein said monitoring circuitfurther comprises: a protection circuit configured to compare a signalrepresentative of a current level associated with said battery pack toan over current threshold and to provide an over current alert signal tosaid processor if said current level is greater than said over currentthreshold.
 21. The electronic device of claim 20, wherein saidprotection circuit is further configured to compare said signalrepresentative of said current level associated with said battery packto a short circuit current threshold and to provide a short circuitcurrent alert signal to said processor if said current level is greaterthan said short circuit current threshold.
 22. The electronic device ofclaim 1, said electronic device further comprising: a battery gaugeconfigured to provide a battery gauge output signal representative ofthe remaining useful life of said battery pack, said battery gaugeconfigured to receive each of said digital signals representative ofsaid voltage level of each said battery and provide said battery gaugeoutput signal based on said digital signals.